I. Field of the Invention
The present invention relates to a method for manufacturing an optical memory disc for recording bit data on a recording film by beam radiation.
II. Description of the Prior Art
Optical memory discs which are capable of recording/reproduction in real time have been recently proposed as a large-capacity high-density memory. In an optical memory disc, bit data is recorded by beam radiation on a disc which is driven at a high speed. The recorded data is then read out by a beam. An optical pickup head is not brought into contact with the optical disc to perform recording/reproduction. Therefore, an optical memory disc of this type has advantages in that the pickup head and the optical disc may not be damaged, and in that the data access can be performed at a high speed.
In general, an optical memory disc comprises a transparent substrate on which concentric or helical pregrooves or projections are formed as tracks, a recording film is deposited thereon by, for example, vapor deposition, and a plastic protective film is formed on the recording film. The thickness of the recording film is controlled by a quartz oscillator gauge. At the time of recording/reproduction, the tracks are scanned with a beam spot under tracking control and focusing control.
Two types of optical memory disc are available at present: a reflecting memory disc and a transmitting memory disc.
In the reflecting memory disc, tracking control and focusing control are performed on the basis of light reflection. When a reflectance varies in accordance with individual discs, it is very difficult to perform tracking control and focusing control. In this case, a deck must be adjusted in accordance with a given optical disc so as to perform optimum tracking and focusing control operations. Therefore, in order to use different discs in a single deck, the reflectances of these discs must be kept at a given value.
On the other hand, in the transmitting memory disc, the transmittances of different discs must also be kept at a given value.
A disc reflectance Rd is a sum of a reflectance Rs of the substrate having pregrooves and a reflectance Rf of the recording film as follows: EQU Rd=Rs+Rf
The reflectance Rf of the recording film depends on the thickness of the recording film, and varies in accordance with an interference effect of the film. However, it is possible to control the film thickness by a quartz oscillator gauge. Therefore, the recording film may have a given reflectance. However, the reflectance Rs of the pregrooved substrate is influenced by the depth and shape of the pregrooves. In particular, when the pregrooves are formed by stamper transfer, the depth and shape of the pregrooves may vary in accordance with the shape of each stamper, and hence the reflectance Rs of the pregrooved substrate varies. Even if the thickness of the recording film is kept at a given value so as to set the reflectance Rf at a given value, the reflectance Rd of the disc varies in accordance with the variation in the reflectance Rs of the substrate. This drawback is also applicable to the transmittance of the transmitting memory disc.